The invention relates to a method for tuning an antenna circuit of an actively transmitting tag to a frequency of an interrogator carrier signal, strictly speaking for retuning after a guest miniature card provided with the actively transmitting tag has been inserted into a host device. The invention also relates to a circuit for carrying out such tuning. Tuning according to the invention is carried out automatically.
An actively transmitting tag (PCT/SI2012/000024) is usually provided with an antenna having very small dimensions, and this is why active transmitting of the tag is applied. Communication of the actively transmitting tag with an interrogator proceeds through a magnetic coupling of their coils, which are components of their antenna circuits with resonance frequencies of e.g. 13.56 MHz according to standard ISO 14443 or ISO 15693. Mutual tuning of tag's and interrogator's antenna circuits to each other increases the strength of signals at their antennas by their quality factors. A distance, at which they can still communicate with each other, gets increased.
Typically, an actively transmitting tag is typically applied in a miniature card, for example a micro SD card or a SIM card. The miniature card is intended to be inserted into a host mobile device such as a mobile telephone, personal digital assistant, tablet PC and similar devices. The actively transmitting tag obtains supply voltage from the host device.
Work environment of the actively transmitting tag is considerably altered when inserted together with the miniature card into a host device. Electrically conductive materials incorporated in a host device reduce inductances in a tag's antenna circuit, the embedded dielectric materials increase antenna circuit capacitances, and parasitic capacitances appear.
A resonance frequency of the tag's antenna circuit is changed by insertion of a miniature card into a host device. Production of miniature cards is normally standardized for all mobile host devices, yet the environment of a connector for a miniature card in host devices strongly varies from one device to another. A producer thus cannot design an actively transmitting tag in a way for its antenna circuit to keep the resonance at the frequency of 13.56 MHz in various mobile host devices.
There is known a miniature card having an elongated coil of a thickness of merely 0.5 mm and provided with a high-permeability core, e.g. ferrite core. The coil is arranged in an antenna circuit in an immediate proximity of and along at least one electrically conductive plate, which does not cross a magnetic axis of the coil (FR 2 965 084, FR 2 965 083). A resonance frequency of the antenna circuit having such configuration is set to a frequency of 13.56 MHz and remains unchanged even after the antenna circuit has been inserted into various mobile host devices.
Indeed, this technical solution has solved the technical problem of how to produce such miniature card that an environment in the host device will not change the resonance frequency of the antenna circuit but it is very hard to produce such coil as well as to provide the miniature card with one or two electrically conductive plates. It is much easier to produce a coil with conductive strips on a printed circuit board of the miniature card, however, a magnetic axis of such coil is perpendicular to a plane of the printed circuit board.